Thoracic aortic aneurysms and dissections (TAAD) are the 15th most common cause of death in the U.S. They are frequently inherited in an autosomal dominant fashion, with the defective gene products including smooth muscle contractile proteins (e.g., MYH11), extracellular matrix (ECM) proteins, and TGF-? signaling components; loss of smooth muscle cells (SMCs) in the aortic media characterizes the disease. We recently identified a single gain-of-function mutation (R177Q) in protein kinase G1 (PKG1) as the cause of TAAD in families with early dissections; the mutation causes constitutive, cGMP-independent enzyme activation. We generated mice carrying the R177Q knock-in mutation (PKG1RQ), which are normotensive and have no overt phenotype up to 4 months of age; however, pressure overload by trans-aortic constriction (TAC) caused excessive mortality with aortic rupture and abnormal aortic and cardiac remodeling. Determining how sustained PKG1 activation affects aortic integrity is of broader clinical relevance, because case reports suggest that repeated use of the PKG-stimulating drug sildenafil (Viagra) can lead to TAAD, and treating mice with a MYH11 mutation with sildenafil increased development of TAAD under hypertensive stress. We found increased reactive oxygen species (ROS) and tissue markers of oxidative damage, excess TGF-? signaling, and altered contractile and ECM-related gene expression in SMCs and aortas of PKG1RQ-mutant compared to wild type mice. We hypothesize that excess PKG1 activity leads to increased oxidative stress and dysregulation of gene expression in SMCs, resulting in impaired aortic wall maintenance/repair, and that reducing ROS production may protect PKG1RQ-mutant mice from aortic aneurysms. In Aim I, we will determine if hetero- and homozygous PKG1RQ mice develop aortic dilation and dissection spontaneously with age, or only under hypertensive stress, and examine the effect of pharmacological PKG activation on aortic wall integrity in wild type and heterozygous PKG1RQ mice. In Aim II, we will compare ROS production by mitochondria, NOX isoforms, and uncoupled NO synthases in wild type and PKG1RQ SMCs and tissues. To determine if excess ROS production is causally linked to aortic pathology, we will treat PKG1RQ mice with cobinamide, a vitamin B12 analog that is a novel ROS neutralizing agent, or cross them with NOX4-deficient mice and mice over- expressing mitochondrial superoxide dismutase or catalase. In Aim III, we will use RNA-seq and gene network analysis to compare a PKG1-induced gene signature in the aorta from wild type and PKG1RQ mice and from patients carrying the PKG1 mutation. We will analyze gene networks known to be involved in TAAD pathogenesis and determine ROS-induced transcriptome alterations. In Aim IV, we will determine the molecular basis of PKG1 activation by the R?Q mutation using deuterium exchange/mass spectrometry. Since PKG1 is the pharmacological target of multiple, clinically-used drugs, a better understanding of the effects of long-term PKG activation and excess ROS on aortic wall integrity is important. The potent ROS- neutralizing agent cobinamide is a promising new agent for anti-oxidant therapy in vascular diseases.